1. Field of the Invention
This invention relates to a synchronism acquisition method and apparatus for use to correlation demodulate a spread code modulated by spread spectrum modulation.
2. Description of the Prior Arts
In spread spectrum (SS) communication based on a direct sequence (DS), an information code is spread modulated, on the transmission side, using a PN code sequence (pseudo-random noise code sequence, PN; pseudo-random noise) to produce a spread code and the spread code is transmitted. Then, on the reception side, the received signal is correlation demodulated (despread) using the same PN code sequence as that used on the transmission side to obtain an information code. The PN code sequence on the reception side is also called despread code. When a spread spectrum is used for communication, synchronism must be established between the PN code sequences of the transmission side and the reception side. However, since generally the generation timings of the PN code sequences on the transmission side and the reception side do not coincide with each other when communication is to be started, synchronism acquisition processing for synchronizing the PN code sequence on the reception side with the PN code sequence on the transmission side must be performed by the reception side upon starting of the communication. Here, the synchronism acquisition processing is processing which is performed at the reception side to estimate a generation timing of the PN code sequence on the transmission side with a predetermined degree of accuracy within one chip.
Also in a frequency hopping (FH) which is one of spread spectrum communication methods together with the direct sequence, synchronism acquisition processing must be performed upon starting of communication to establish synchronism between a frequency hopping sequence on the transmission side and a frequency hoping sequence on the reception side.
It is to be noted that processing for establishing synchronism between PN code sequences or frequency hopping sequences includes a synchronism acquisition process for establishing coarse synchronism which is described herein and a synchronism tracking process for establishing fine synchronism after the coarse synchronism is established and for finely adjusting the generation timing of a despread code sequence so that the established synchronism may not be lost.
A basic method of the synchronism acquisition process is a method of receiving a PN code sequence from the transmission side while successively varying the generation timing of a PN code sequence on the reception side little by little to effect correlation demodulation to find out a generation timing at which a high correlation value is obtained, that is, a sequence search method. With this method, however, a rather long time is required to acquire synchronism where the S/N (signal-to-noise ratio) of a transmission line is not high or a like case, and therefore, several techniques for performing synchronism acquisition processing at a high speed have been proposed. For example, Japanese Patent Laid-Open Application No. Hei 3-3530 (JP, A, 3-3530) by Y. Komatsu discloses a correlation demodulation apparatus wherein a primary search is performed for a full search range to find, as candidates, a plurality of chip phases which have relatively high correlation values and then a secondary search is performed to integrate the candidate chip phases by a plurality of times so that a chip phase synchronized with that of the transmission side can be found out in a comparatively short time.
FIG. 1 is a block diagram showing a construction of the correlation demodulation apparatus disclosed in above JP, A, 3-3530. The correlation demodulation apparatus shown in FIG. 1 includes three correlation detectors 2 to 4 to which a received signal 1 is inputted. The correlation demodulation apparatus further includes a clock generator 13 for generating a clock signal, a PN code generator 5 for generating a PN code sequence in response to the clock signal, and a shift register 6 for shifting the phase of the generated PN code sequence by +1/2 chip and -1/2 chip. The PN code sequence from the shift register 6 which has no displacement in phase is supplied to the correlation detector 2 while PN code sequences which are shifted by +1/2 chip and -1/2 chip in phase are supplied to the correlation detectors 3 and 4, respectively. The correlation detectors 3 and 4 are provided to effect synchronism tracking processing (fine synchronization). The correlation demodulation apparatus further includes reset integrators 7 to 9 provided on the output sides of the correlation detectors 2 to 4, respectively, registers 10 to 12 for holding the outputs of the reset integrators 7 to 9, respectively, and a controller 14 for controlling the entire apparatus.
In the conventional correlation demodulation apparatus shown in FIG. 1, synchronism acquisition processing is performed in the following manner. A PN code sequence is generated successively delaying it by one chip until a phase delay corresponding to one repetition period of a code sequence is produced, and the PN code sequence is inputted to the correlation detector 2 and correlation values (i.e., reception levels) then are calculated by the reset integrator 7 and successively stored into the register 10. In this manner, correlation values for one repetition period of the code sequence which individually correspond to the chip phases of the PN code sequence are stored into the register 10. Then, as the primary search, chip phases corresponding to the correlation values which have comparatively high values and are stored in the register 10 are selected as candidates. Next, as the secondary search, the correlation values are integrated with the phases corresponding to the candidates and it is determined that synchronism is acquired when the integrated value exceeds a predetermined determination level.
After the above synchronism acquisition processing, the correlation demodulation apparatus subsequently performs synchronism tracking processing.
With the conventional synchronism acquisition method which involves a primary search and a secondary search, since a primary search is first performed for a full synchronism acquisition range and then correlation detection and correlation integration are performed again, in order to raise the S/N ratio, for all candidates selected by the primary search to calculate correlation values to find out an optimum chip phase, there is a problem in that much time is required to establish chip synchronism. Further, since correlation values are calculated at the intervals of one chip for the full synchronism acquisition range, where a peak is present, for example, at a position spaced by 1/2 chip, this peak cannot be detected. Therefore, the conventional synchronism acquisition method has another problem in that a peak of a correlation value cannot be detected accurately for the full synchronism acquisition range.